Solar-driven splitting of water (H2O) into hydrogen (H2) and oxygen (O2) using photoactive catalysts is one of the “Holy Grails” of science (see the article entitled: “Artificial Photosynthesis: Solar Splitting of Water into Hydrogen and Oxygen” by Allen J. Bard and Marye Anne Fox, in the journal “Accounts of Chemical Research (1995), Vol 28, pages 141-145). Efficient and sustained water-splitting using nothing but sunlight has the potential to solve two of mankind's most pressing problems, namely:                (i) Large-scale electricity and/or fuel generation.                    Hydrogen is a fuel that could, in future, displace gasoline and diesel in transportation and other applications. Hydrogen and oxygen are, for example, already used as a rocket fuel. Hydrogen and oxygen can be readily recombined in a fuel cell to generate electricity. Several tens of terawatts of solar energy arrive at the Earth's surface each year. If efficient means were available to collect and convert even a tiny fraction of this energy into hydrogen and oxygen, it could supply the entire energy needs of the current and predicted future population of the planet.                        (ii) Climate change due to greenhouse gas effects.                    Water-splitting generates the gases hydrogen and oxygen that do not have known greenhouse effects. No carbon emission is involved.                        
Despite several decades of intensive work in the area, a photocatalytic system that effectively splits water using only sunlight is still absent. The main reason is that water is one of the most stable compounds on Earth.
The process of splitting water is known as “electrolysis”. It involves two half-reactions that must occur at the two electrodes in an electrochemical cell with water as the electrolyte. The water is split into oxygen, O2, at one electrode (“water oxidation”) and hydrogen, H2, at the other (“proton reduction”), according to equations (1) and (2).2H2O→O2+4H++4e−  (1)2H++2e−→H2  (2)
Water-splitting is an endothermic reaction and requires the provision of energy to proceed spontaneously. This energy can be provided from an external electrical power source or it could be created by some other means. Because an external electrical voltage must be applied to drive the reaction, electrolysis is unviable as a means of cheaply generating hydrogen. If, however, the voltage could be created by some other means, such as by illumination with sunlight, then H2 could be potentially generated cheaply. The key problem is the water oxidation step (equation (1)), which has a higher activation energy than hydrogen generation (equation (2)). Only one truly efficient photocatalyst of water-oxidation is known—the biological Water-Oxidizing Complex (WOC) in Photosystem II (PSII) that is found in all photosynthetic organisms.
As can be seen in equation (2), the step of oxidizing water, H2O, requires:                the removal of 4 electrons from two water molecules,        the breaking of 4 chemical bonds, between hydrogen and oxygen atoms (two H—O bonds in each water molecule), and        the formation of 1 new bond between two oxygen atoms (to give O2).        
This step can only be viable if it is facilitated by powerful catalysts that greatly decrease the activation energy involved in water oxidation. Catalysts are species that mediate chemical reactions without themselves becoming changed. Precious metal catalysts, like platinum (Pt), are typically used in commercial water electrolyzers to oxidize water and/or reduce protons by the application of an external voltage bias (usually >2 V). Whilst such catalysts are not light-driven, they are, at least, relatively energetically efficient in facilitating the water-splitting half-reactions.
For the step of water oxidation to be driven by light, one needs a photosystem in which photoinduced charge separation is followed by an efficient electron cascading step that results in the oxidation of water. Catalysts that facilitate the latter reaction when illuminated by light are critical. Several potential catalysts of this type exist, however the conditions involved in oxidising water are so demanding that most light-driven catalysts decompose over time, leading to photo-degradation and the eventual termination of the water-splitting reaction.
The proton reduction step (equation (2)) is, by contrast less energy intensive. It requires only:                the addition of 2 electrons to two protons (H+),        the forming of 1 new bond between two hydrogen atoms to yield H2         
“Solar-driven” water-splitting, in which the only components required are sunlight, a photocatalyst and water, is distinguished from “solar-assisted” water-splitting where in addition to illumination by light, one also applies a moderate voltage bias using an external power source or electricity grid. Alternatively, a chemical bias may be applied in addition to the light illumination, by employing sacrificial additives to oxidize or reduce the intermediates in the catalysis. The additives may also be used to increase or decrease the pH.
Many solar-assisted water-splitting systems may be considered to be “half-cells”, since only either the water oxidation or proton reduction side is efficient, but not both. The electrical or chemical bias is used to facilitate the inefficient reaction to at least the same level as a comparable solar-driven reaction so it does not limit the overall performance of the water-splitting cell.
A common approach in water-splitting research is to employ a so-called photoelectrochemical cell (PEC). Such cells comprise two electrodes—an anode and a cathode—separated by water electrolyte. The anode facilitates the water oxidation step (equation (1)) and is typically light-responsive. This electrode usually absorbs photons of light to induce charge separation. The resulting electrons travel through an external circuit to the cathode. As a result, so-called “holes” are left within the photoanode material itself. These holes, which have sufficient electrochemical potential to oxidise water, migrate to the surface of the photoanode and convert water into oxygen. The photogenerated electrons travel via an external circuit to the cathode where they reduce protons, generating hydrogen.
In such a system, the photoanode is the “motor” that drives the overall reaction. There are a number of potential photoanode materials available, including semiconductors like TiO2, Fe2O3, WO3, and CdSe. However, all such materials suffer from serious technical problems that limit their practical utility, including:                (i) they are insufficiently photo-stable under prolonged illumination and are therefore unsuitable for long-term operation;        (ii) their visible light response is limited to wavelength regions that comprise only small proportions of the solar spectrum. They, therefore, are inefficient at harvesting the energy of the illuminating light;        (iii) they can sometimes only be used for half cells. That is, their driving force under illumination by sunlight is insufficient to facilitate overall water-splitting at both electrodes.        
To overcome or ameliorate the issue of photo-stability and visible light-response, large band gap inorganic oxide semiconductors like TiO2, ZnO, SnO2, Nb2O5, may be “sensitized”. To improve the efficiency with which they harvest the energy from sunlight, a dye is attached to the semiconductor anode. The dye absorbs a larger proportion of the light than the underlying semiconductor. Upon absorption of a photon of light, the dye injects an electron into the semiconductor. The oxidised dye is regenerated when it oxidises water according to equation (1). The injected electrons move through the external circuit to reduce protons at the cathode according to equation (2). This process is referred to as “n-type dye-sensitisation”.
Sensitization is a technique that comes out of the field of “Dye-Sensitised Solar Cells” (DSSC's) (also referred to as: dye-sensitized photovoltaic cells). DSSCs generate an electrical voltage when illuminated with sunlight. Sensitizers are used to expand the wavelength regions of the solar spectrum that are absorbed by the solar cell.
There are very significant problems with using n-type dye-sensitisation for solar-driven water-splitting or for solar-driven reactions that employ a water-based electrolyte. The major problem is that water interferes with many of the reactions that are employed in conventional dye-sensitized solar cells. Additionally, the following key issues present themselves in the specific case of water-splitting:                (i) The oxidation potential of the dye needs to be larger than the oxidation potential of water, typically +0.817 V to +1.23 V (at pH 7 and 0, respectively). In the absence of strong catalytic activity of the dye itself, a large (0.5 V to 1 V) overpotential is required to effectively drive this reaction. The number of dyes that have such a high oxidation potential and are able to perform all the other functions, such as visible light absorption and charge injection, is vanishingly small.        (ii) Using a co-catalyst may reduce the overpotential for water oxidation, however it introduces another problem—that of interfacing the light harvesting unit with the co-catalyst. While the water oxidation overpotential may, thus, be diminished, a new electron transfer step is introduced between the light harvesting dye and the co-catalyst, which requires a driving force to proceed spontaneously and efficiently. This is lowering the maximum achievable efficiency of this process.        (iii) Under such strongly oxidising conditions, the dye or the dye/semiconductor interface is typically unstable. Most dyes absorbing in the visible spectral range contain unsaturated double bonds, which are prone to oxidation under strongly oxidising conditions, leading to instability.        (iv) The water oxidation reaction generates protons that may catalyse the desorption of the acid-linker groups used in most typical dye sensitiser linkers. Dyes that do not desorb rapidly from the semiconductor surface are yet to be developed.        
At present, three dye-sensitised photoelectrochemical systems have been reported that can oxidise water using a co-catalyst. All employ n-type sensitization. These devices display 1-2% photon-to-electron conversion efficiencies, and feature very short device lifetimes, in the realm of minutes. All operate under “half-cell” conditions where the hydrogen is generated at a platinum (Pt) counter electrode by applying a moderate, additional voltage bias.
Consequently, a clear need exists to develop improved solar water-splitting devices which address or at least ameliorate one or more problems inherent in the prior art.
A clear need also exists to develop solar cells capable of undertaking chemical transformations in water-based solvent systems. There is a general move to “green” chemistry, which avoids the need for organic solvents in chemical reactions. This is also true for solar-driven chemical transformations.
The reference in this specification to any prior publication (or information derived from the prior publication), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that the prior publication (or information derived from the prior publication) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.